The present invention relates to electronic timers useful for timing and controlling processing operations in a processing system and for indicating timing information to an operator.
In many processes it is desirable or necessary to vary or to terminate the process for a period of time upon the occurrence of a certain event. For example, when a submergible motor-driven pump is employed in a downhole petroleum reservoir in an oil well for pumping liquid to the surface, if the liquid is pumped from the reservoir faster than the rate at which it enters, eventually the liquid level will decrease below the level of the pump intake. When this occurs, it is desirable to shut down the pump until the liquid is restored to a level sufficient for pumping. This may require several minutes or several hours depending upon various factors. Since oil well pumping systems normally monitor one or more process variables associated with the motor/pump, such as flow or motor current, a drop in liquid level below the pump intake may be readily detected and the pump may be shut down automatically. After a predetermined period of time has elapsed which is sufficient for the reservoir to refill, the pump may be restarted. A timer is normally used for measuring the pump down-time. The timer may be associated with a control system or it may be used simply for signaling an operator to restart the pump manually. Preferably, the timer is variable so that the amount of time that the pump is shut down can be varied as desired, and, preferably, it is capable of indicating timing information to the operator so that he knows how long he must wait for the pump to be restarted.
Timers are known which may be employed for these purposes. However, known timers have several disadvantages. Mechanical timers tend to be cumbersome and must be manually reset after each timing operation. Their accuracy may vary significantly with temperature and other environmental factors, and they are difficult to seal to prevent corrosion, etc., which may affect their operation.
Electro-mechanical timers which employ a small electrical timing motor for advancing a pointer through gears until a predetermined time has elapsed may also be used. Resetting of the timers is usually automatic, and they are generally more accurate than mechanical timers. However, they also tend to be large and present sealing difficulties which are similar to those in mechanical timers. Moreover, since the timing motor is usually wound with a large number of turns of very fine wire, it is susceptible to insulation failure when used on power lines which are subjected to lightning induced surges.
Available electronic timers avoid some of the above problems but tend to be complicated and expensive devices. Also, they normally must be purchased as a stand-alone device.
It is desirable to provide indicating electronic timers which overcome these and other disadvantages of prior art timers, and it is to this end that the present invention is directed.